Acute oak decline (AOD): Incidence and distribution

The incidence of acute oak decline is on the increase as it spreads across Britain

Acute oak decline (AOD) is a new disease mainly affecting native oak trees in Britain and considered to have first made a presence in Britain 30‑35 years ago. It is most prevalent on English or pedunculate oak (Quercus robur) and sessile oak (Quercus petraea).

Recent spatial epidemiology studies show that the disease can spread from tree to tree and so it is necessary to know where it occurs at the landscape level as well as its distribution within sites. With this knowledge in hand, we can map the distribution of AOD, monitor spread of the disease and predict at risk areas. Then through proactive management, prevent or curb the spread of the disease to unaffected areas and protect trees not yet affected.

This work area is funded in part by the Forestry Commission and in part, by Defra. Collaborative work with Rothamsted Research and Cambridge University is underway.

Forest Research holds a database on symptomatic oak trees reported to the Disease and Diagnostic Service (DDAS). Reports of symptomatic trees are collected and where possible, verified. Collation of these reports has allowed the known distribution of AOD to be mapped (See distribution maps below).

AOD was first defined in 2008/9 in Britain, but photographic and descriptive data are available from 2006 onwards and so these early reports were assessed retrospectively. Over time the public awareness of AOD has increased and this has resulted in an increased number of reports made to Forest Research. When plotted as annual maps the distribution of confirmed affected sites can be seen to increase in southern England, with it now covering a band across the Midlands from East Anglia to the Welsh borders. Initially verification was very labour intensive but with the recent development of a rapid diagnostic tool the verification process has been quickened. FR is in the process of verifying as many AOD reports as possible.

Current distribution of acute oak decline (all sites verified)

Map updated to March 2016

Distribution 2006 – 2014

Reports from landowners and managers provided a vital first impression of the extent of AOD across the country and remain an important tool for early detection. However, reports are more likely where woods are frequently visited and are dependent on public awareness of the problem. To obtain a more accurate assessment of the true situation and to identify areas not yet affected, a systematic survey is underway. In the summers of 2013 and 2014 oak woods across England and Wales were visited by foresters and surveyors from Forest Research.

The overall aim of this work area is to improve understanding of the current distribution of affected sites. By combining these data with epidemiological modelling the project aims to elucidate any environmental factors or processes that underlie the observed pattern and thus allow projections of the risk of future spread.

This project is a collaborative process between FR, Rothamsted Research, Cambridge University and the Forestry Commission’s National Forest Inventory group, but has also involved the help and co-operation of many landowners, managers and stakeholders including the National Trust, Woodland Trust, various Wildlife Trusts, and local councils, whom we thank. It is jointly funded by the Forestry Commission and Defra.

Predictive Modelling

By using the data collected from FR’s DDAS reports and the survey, epidemiological models on the spread and establishment of AOD in Great Britain are being developed. This work forms part of the Defra project (hyperlink) and is being carried out in collaboration with Cambridge University and Rothamsted Research.

The models aim at complementing the available reports and survey by providing:

Estimates of the distribution of AOD in currently unsampled areas

The time of arrival of AOD across its geographic extent (the arrival will have occurred at an unknown time prior to reporting or survey, where these are available).

In addition, the epidemiological models will allow:

Estimation of the current rate of spread and areas at greater risk, as well as the future spatial distribution of AOD and its uncertainty

Testing hypotheses about the mechanisms of dispersal and driving environmental factors that underlie the spread and development of AOD

Exploring changes in the risk of AOD under a variety of scenarios, including variation in environmental conditions and implementation of various management practices